1. Field of the Invention
The invention pertains to an apparatus for performing fatigue and/or crack growth tests including complex loading with regard to the relative magnitude and waveform of the load cycles, with the ability to apply axial tension, compression, and/or torsional loading independently, potentially resulting in fully mixed-mode crack growth with non-proportional loading. The novel elements of the invention include both the device for application of the loads (the test machine), and a test sample configuration that the machine is especially suited to test.
While resonant (dynamic) conditions may be possible to achieve with the apparatus, an important object of the invention is to extend the advantages of closed-loop, non-dynamic testing to moderately high frequency ranges. A further object is to achieve high quality test capability in a miniature, low cost apparatus. Another object is to provide a crack growth test option with a uniform plasticity-induced closure state across the crack front.
These and other objects, advantages and characteristic features of the present invention will become more apparent upon consideration of the following description thereof when taken in connection with the accompanying drawings depicting the same.
The subject of signal generators and power electronics required to drive the subject apparatus is a peripheral field not directly relevant to the invention.
2. Description of the Prior Art
Fatigue and crack growth testing is necessary in many engineering applications where component durability and safety concerns merit the associated costs. It is often desirable to increase the frequency of such testing to more closely simulate field conditions, as is particularly true for high-cycle fatigue or crack growth threshold testing. Nevertheless, high frequency testing is also desirable if for no other reason than to reduce the duration and cost of testing. The most common fatigue test machines apply cyclic load to a sample mounted between two connection points with cyclic loading supplied by servohydraulic, or servoelectric actuation systems, and seldom exceed 100 Hz frequency capability due to inherent design limitations. However, it is not uncommon for these machines to employ closed loop load, displacement, or even crack tip stress-intensity control capable of arbitrary load waveforms and complex loading sequences, which can be very desirable in some applications. The use of these types of machines and the common samples employed for fatigue and crack growth testing are described by ASTM standards (especially ASTM E466 and E647) and is well known to those familiar with the art.
Another common class of cyclic loading machines, particularly in connection with higher frequency loading up into the acoustic range using servoelectric actuation, involves specimens supported by connection to a single actuating interface and cycled dynamically, typically to interrogate or excite a resonant condition. Such shaker or vibrator table devices rely on resonance to impart sufficient loading within the sample tested to bring about damage or failure, and are typically incapable of achieving fatigue failure or crack growth at frequencies distant from a resonant condition. Further, while the dynamic excitation signal can in principle be complex in nature, the response will be limited by the dynamics of the sample, and thus an arbitrarily defined load waveform or complex load sequence is not possible. Cracks growing in dynamic tests can also cause detuning and loss of load amplitude.
The same limitations apply to an ultrasonic piezoelectric dynamic system operating at 15-30 MHz as described in Gigacycle Fatigue in Mechanical Practice, by Paul C. Paris and Claude Bathias CRC Press 2004.
A more recent and less common class of test machines includes machines that are loaded from two independent load interfaces at high frequency.
An example of such a device is described in German publication DE 28 29 858, which employs an electromagnetic actuation mechanism to excite at frequencies up to 1000 Hz. The sample is connected to a fixed connection at one end, and attached to a magnetic conductor at the opposite end, which is separated from an electromagnet by an air gap. A cyclic attractive/repulsive field creates the forces that load the specimen, but is limited to simple cycles.
GB 2 060 179 A describes a high frequency cyclic test machine with load connection at each end of the test sample, loaded by a stacked piezoelectric actuator within a rigid frame. While the primary embodiment has a mass and spring arrangement for resonance, this can be removed to operate in closed loop control. It also includes a prestressed enclosure for the piezoelectric actuator for the purpose of preloading it in compression, a feature common to most applications involving piezoelectric or piezoceramic actuators, as a protection to the actuator material which is weak in tension. For dynamic operation this is of minor consequence because the loading is amplified by resonance. In GB 2 060 179, however, the prestressing member must be operated at a substantial load if the specimen is to be cycled in tension, thus consuming much of the load producing capability of the piezoelectric actuator, which is especially critical in non-resonant closed loop mode, and invites failure of the prestressing member under high cycle fatigue loading.
U.S. Pat. No. 6,023,980 combines independent servohydraulic closed loop operation and dynamic piezoelectric excitation to enable more general loading conditions with high frequency loading.
U.S. Pat. No. 7,770,464 B2 offers one embodiment with an improved actuator arrangement wherein a tensile load in the sample is reacted as compression in at least some of the piezoceramic actuators, allowing the actuators to command a tensile load directly, but retains a prestressing element, with its small, but nevertheless parasitic secondary load path. However, a severe drawback of this concept, with regard to the object of the current invention, is that the prestressing element, which is necessarily a compliant member, serves not only as a prestressing element, but remains the sole load path for tensile load, short of putting the actuators in tension. Again, this is tolerable with the actuator concept being expressly intended for dynamic operation. But not for the current object. Also described are concepts for simultaneous axial and torsional loading.
U.S. Pat. No. 6,020,674 describes a torsional electroactive actuator, such as might be employed as a prior art component in the current invention.
With regard to prior art in sample geometries for fatigue and crack growth testing, the most commonly used configurations are described in the ASTM standards referenced previously, with several other potential test configurations described in stress intensity handbooks such as the Stress Analysis of Cracks Handbook, 3rd Ed (H. Tada et al, ASME press, 1997). The compact tension specimen is of particularly common usage for crack growth, but is well known to have non-uniform plasticity induced closure across the crack front, resulting from reduced constraint in the vicinity of the intersection between the crack front and the free surface. Specimens with quarter circular or semicircular cracks are also popular, and have the benefit of resembling common naturally occurring crack shapes, but are also subject to free surface effects, though to a lesser degree. Free surface effects are absent in samples with a fully circular crack front, such as circular cylindrical or tubular specimens with a circumferential crack loaded in tension. However, because the stress intensity increases with crack length for these configurations, any deviation from a truly concentric crack front within the specimen is augmented as the crack grows, resulting in crack front shape instability. This hampers correlation of the data with a single stress intensity solution, and impairs the reproducibility of results.